We wish to characterize the manner in which RNA polymerases interact with and discriminate among transcription signals in the DNA template. Because the RNA polymerase encoded by bacteriophage T7 is structurally one of the simplest RNA polymerases known, this enzyme provides an excellent model system with which to study this process at the molecular level. Evidence that we have accumulated thus far indicates that this enzyme discriminates between two classes of promoters that differ in the ease with which they are melted open by the RNA polymerase. To regulate late transcription, the virus alters the melting properties of the DNA helix during infection, possibly by altering the supercoiled state of the DNA. This mechanism of gene regulation has obvious implications to studies of regulation in eukaryotic cells. To study this problem, we will: (a) Manipulate the structure of T7 promoters in vitro and determine the effect of the changes on promoter utilization in vivo. (b) Alter the supercoiled state of the viral DNA in vivo through the use of appropriate drugs and/or viral mutants.